1. Field
The disclosed embodiments relate to the field of non-destructive inspection of parts by ultrasound reflection.
In particular, the disclosed embodiments relate to a method suitable for automatically inspecting non-flat areas or areas with non-parallel faces of parts such as parts made of composite materials.
2. Brief Description
The general principle of non-destructive ultrasound inspection of structural parts is these days well known and widely implemented.
This principle consists in placing all or some of a part to be inspected in an immersion medium that is a good conductor of ultrasound acoustic waves, for example by immersing the part in a tank or by locally creating immersion conditions by means of a water box or even by creating a fluid immersion film by the continuous application of the fluid, this immersion medium being more often than not water because of its excellent acoustic wave conduction characteristics, its low chemical aggressivity for the inspected parts and its negligible cost, in emitting ultrasound waves towards points of the part to be inspected and collecting the reflected waves in the form of echoes from the part. In practice, the waves are reflected by the different interfaces of the part each time a discontinuity provokes a reflection of a portion of the energy of the emitted acoustic wave. These discontinuities are more often than not a wall of the part, at the interface with the immersion medium or with a joined part, a defect in the material of the part. The reflected waves are also influenced by characteristics of the material passed through. Thus, for example, its porosity can be assessed by measuring the attenuation of the waves reflected by a rear interface, provided that the waves reflected by said rear interface reach measurement sensors.
The ultrasound acoustic wave is propagated in the medium in which the part is immersed and in the material of the part and, according to the known laws of propagation and reflection, each time the incident acoustic wave encounters a discontinuity or a variation of the characteristics, a portion of the incident wave is transmitted and another portion is reflected.
The energies and the phases of the transmitted and reflected waves make it possible, after signal measurements and processing, to determine certain characteristics of the part being inspected.
When applied to parts made of composite materials, the non-destructive ultrasound inspection can be used to detect internal defects in the parts such as, for example, delaminations or porosity characteristics which are essential parameters for parts subject to stresses in their use.
The general principle of the means used for these non-destructive tests is illustrated by FIG. 1a. 
A probe 2 emits in an emission mode an ultrasound wave 3 towards a part to be inspected 1a. A surface 11a of the part la which receives the incident wave 3 reflects a portion of this incident wave in the form of a first reflected wave 4 which is measured by the probe 2 then working in reception mode. Another portion of the incident wave which penetrates into the material of the part 1a is in turn reflected by another surface 12a of the part and forms a second reflected wave 5 which is also able to be measured by the probe 2 working in reception mode.
To perform this type of measurement, probes that emit the incident waves and that measure the reflected signals are normally used. These probes more often than not use transducers based on piezoelectric technologies which are perfectly suited to the frequencies of the ultrasounds used and which have the advantage of being able to measure the reflected waves with the same transducers as those that generate the incident wave.
However, the need for high spatial resolution means selecting directional sensors, which sensors are ineffective when one of the interfaces of the part, an input or bottom interface, is not normal to the direction of propagation of the incident acoustic wave.
As illustrated in FIG. 1b, if the surface 11a of the part 1a is not perfectly perpendicular to the direction of the incident wave 3, the reflected waves 4 and 5 will not be directed efficiently to the probe and the latter will not be able to return a satisfactory measurement. This phenomenon is not too problematic when the probe is almost in contact with the surface of the part, as in the case of probes held by an operator, but proves critical when the probe is distant from the surface of the part which is normally the case in automated inspection devices with an immersed part. The probe in these devices is located at a distance more often than not of between 3 and 12 centimeters and a very small relative positioning deviation between the part and the probe is sufficient to disturb the measurement, one degree of angular deviation being sufficient to lower the amplitudes of the received signals by several decibels.
When, as represented in FIG. 1c, the part 1b has non-parallel faces 11b, 12b, a correct relative positioning of the probe and relative to the front face 11b of the part is no longer sufficient to ensure a correct measurement of the wave 5 reflected by the rear face 12b and, in this case, the existing devices often require further inspections to be carried out.
Thus, the highly directional probes, using a sensor or a linear array of sensors implemented with a sequential sweep mode to orient the axis of the ultrasound beam, are always difficult to implement because of their very low tolerances to the positioning deviations relative to the parts to be inspected, particularly in automated inspection processes and in particular when the parts to be inspected have a complex or disturbed geometry.